In this specification, the term "coin" is used to encompass genuine coins, tokens, counterfeit coins and any other objects which may be used in an attempt to operate coin-operated equipment.
Coin testing apparatus is well known in which a coin is subjected to a test by passing it through a passageway in which it enters an oscillating magnetic field produced by an inductor and measuring the degree of interaction between the coin and the field, the resulting measurement being dependent upon one or more characteristics of the coin and being compared with a reference value, or each of a set of reference values, corresponding to the measurement obtained from one or more denominations of acceptable coin. It is most usual to apply more than one such test, the respective tests being responsive to respective different coin characteristics, and to judge the tested coin acceptable only if all the test results are appropriate to a single, acceptable, denomination of coin. An example of such apparatus is described in GB-A-2 093 620.
One particular test which is often applied is to determine the maximum effect that the coin has on the amplitude of a signal derived from the inductor. This may be done simply by measuring the peak value that the amplitude reaches as the coin passes by the inductor, or measuring both that peak amplitude, and also the amplitude when the coin is not adjacent to the inductor and taking a function of (for example, either the difference between, or the ratio of) those two amplitudes so as to obtain a value which is less influenced by drift in the circuitry and variations in component parameters. These tests based on amplitude give an indication of the effective resistance (or loss) that is introduced into the inductor circuit by the coin when the coin is sufficiently close to the inductor that eddy currents are being induced in it.
In EP-B1-0 062 411 there is disclosed a method of testing coins in which, as one feature, the effective resistance or loss of a coil, as influenced by a coin held stationary adjacent the coil, is measured by switching a phase change repeatedly into, and out of, the feed back loop of an oscillating tuned circuit, measuring the oscillation frequency with the phase change in the circuit, and without the phase change in the circuit, and taking the difference between the two measured frequencies as an indication of effective resistance. It is inherent in that method that frequency measurements have to be taken on the same coin, using the same circuit, but at different times. To enable this to be done, EP-B1-0 062 411 proposes that after the arrival of a coin in the testing apparatus has been detected a delay of one third of a second is provided to allow the coin to come to rest in a fixed stable position against a stop in a coin runway, where the coin is located between the two halves of a testing coil. When the coin is in that fixed position, the phase change is repeatedly switched into and out of the oscillator circuit for periods which are at least 3.75 ms long, and this is done many times whilst frequency measurements are taken, the coin then being released by the stop to continue its passage through the testing apparatus.
Although in principle this is a useful way of measuring resistance or loss, in practice the need to hold the coin stationary makes the method and apparatus unsuitable for testing a succession of coins rapidly one after the other, which is a requirement in most practical applications of coin testing apparatus.